Currently, if the bot_update script fails, then recipe code that makes
assumptions about the returned output JSON cause exceptions to be
raised. This fixes a couple instances in the code so that the bot_update
failure should surface instead of an uncaught exception.

Change-Id: I031e613841b03e5eb4f6d9ca7adb5085088a3077
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2405708
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>